1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device having an internal power supply voltage dropping circuit.
2. Description of the Related Art
In recent years, as the microfabrication of a MOS transistors (MOS field-effect transistors) has progressed, gate oxide film thickness must be reduced in accordance with the scaling rule. Thus, since the electric field applied to the gate oxide film must be relaxed, an internal power supply voltage dropping circuit (to be referred to as a voltage dropping circuit hereinafter) is used in order to set the power supply voltage used in a chip lower than the external power supply voltage (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 5-159572 (FIG. 2 and the like)).
The operating principle of the voltage dropping circuit will be described below. The internal power supply voltage dropped by a power supply current supply transistor is monitored, and the monitored internal power supply voltage is compared with a reference voltage generated by a reference voltage generation circuit (to be referred to as a reference voltage circuit) inside the chip. In accordance with the comparison result, the power supply current supply transistor is negatively fed back to maintain the constant internal power supply voltage.
However, a negative feedback circuit must respond at a certain speed or more, and steadily pass a current at a certain magnitude or more. Hence, it is difficult to design a product whose current consumption must be low, thus posing a problem. Even in the above-described voltage dropping circuit, the response of a feedback loop deteriorates when reducing the current flowing into a negative feedback circuit. Therefore, the internal power supply voltage readily fluctuates, or oscillates in the worst case. More specifically, since the circuit operates differently from normal operation at the leading edge of the external power supply voltage, the circuit operates unstably. For this reason, the internal power supply voltage easily oscillates. Once the internal power supply voltage oscillates, the oscillation leads to continuous oscillation and an operation error, thus posing a problem.